1. Field of the Invention
The present invention relates to an oxymethylene polymer resin shaped article having high flexural modulus. More particularly, the present invention is concerned with an oxymethylene polymer resin shaped article which is produced by molding an oxymethylene polymer resin, wherein the oxymethylene polymer resin is selected from the group consisting of an oxymethylene homopolymer resin, an oxymethylene copolymer resin and a mixture thereof. The oxymethylene copolymer resin comprises a copolymer of an oxymethylene monomer and a comonomer copolymerizable therewith, wherein the amount of the comonomer is extremely limited. The shaped article has an improved crystal structure, wherein the crystallinity is 72% or more, the average crystallite size is 150 .ANG. or more, and 70% by volume or more of the whole volume of the shaped article is comprised of spherulites each having a diameter of 60 .mu.m or less. The shaped article also has a thickness of 1 mm or more. The oxymethylene polymer resin shaped article of the present invention exhibits excellent mechanical properties including especially high flexural modulus.
The oxymethylene polymer resin shaped article of the present invention can be advantageously used as a material for a sliding part and a precision part. Further, the oxymethylene polymer resin shaped article of the present invention exhibits high flexural modulus and high heat conductivity both at high temperatures, and, hence, can be advantageously used in the field of parts for electric and electronic apparatuses.
2. Prior Art
A shaped article of an oxymethylene polymer resin has various excellent properties, such as excellent mechanical properties, excellent fatigue resistance, excellent self-lubricating properties, and excellent heat resistance. Therefore, oxymethylene polymer resin shaped articles have been used in a wide variety of commercial applications, such as various parts for use in various machines, for example, automobiles, electric apparatuses, electronic apparatuses and office automation machines.
These commercial applications include those applications in which an oxymethylene polymer resin shaped article is required to exhibit especially high mechanical properties while maintaining the other excellent properties inherent in an oxymethylene polymer resin shaped article. In such applications, the mechanical properties of an oxymethylene polymer resin shaped article are improved by a method in which, before the molding of an oxymethylene polymer resin, a large amount of a reinforcement (filler), such as a glass fiber or a carbon fiber, is incorporated into the oxymethylene polymer resin. However, with respect to the resultant composite shaped article having incorporated thereinto a large amount of a reinforcement, there have been problems in that the shaped article has a large specific gravity, a poor sliding property and a poor surface appearance.
Therefore, various attempts have been made to improve the mechanical properties of the oxymethylene polymer resin shaped article by a method using very little or no reinforcement. Among these attempts, there have been attempts to achieve the above object by improving the crystal structure of the oxymethylene polymer resin. As examples of known methods for improving the crystal structure of the oxymethylene polymer resin in an attempt to obtain a shaped article having improved mechanical properties, there can be mentioned a method in which an oxymethylene polymer resin to be molded is subjected to isothermal crystallization at a relatively high temperature, and a method in which an oxymethylene polymer resin to be molded is subjected to annealing (see, for example, J. of Appl. Polym. Sci. vol. 1, p. 169 (1959); J. Macromol. Sci. Phys. B13(3), p. 323 (1977); Polymer vol. 19, p. 1163 (1978); and J. of Appl. Polym. Sci. vol. 55, p. 489 (1995)). The oxymethylene polymer resin shaped articles obtained by these methods are remarkably improved in crystallinity; however, these shaped articles are disadvantageous not only in that the degree of the improvement in the flexural modulus is small, but also in that the extensibility and toughness are reduced. Therefore, these oxymethylene polymer resin shaped articles cannot satisfactorily meet the needs of the commercial applications.
As further examples of known methods for improving the crystal structure of the oxymethylene polymer resin so as to improve the mechanical properties of the shaped article obtained therefrom, there can be mentioned a method in which a crystal nucleating agent is added to an oxymethylene polymer resin to be molded, and a method which utilizes a phenomenon such that when an oxymethylene polymer resin is heated to a temperature slightly higher than the melting temperature of the polymer resin, a non-melted crystal nucleus remains in the polymer resin and serves as a crystal nucleating agent. This facilitates the crystallization of the polymer resin, wherein the non-melted crystal nucleus is presumed to be a portion still having a slight regularity in structure or having an unknown specific structure. Hereinafter, this phenomenon is frequently referred to simply as "self-nucleating phenomenon" (see, for example, Polymer vol. 20, p. 1470 (1979); and Polymer Bulletin 24, p. 445 (1990)). In the oxymethylene polymer resin shaped articles obtained by these methods, the formation of spherulites having a very small diameter can be achieved. However, by these methods, it is difficult to obtain shaped articles having a remarkably improved flexural modulus. Therefore, the oxymethylene polymer resin shaped articles obtained by these methods are not always satisfactory.
Further, Polymer vol. 29, p. 793 (1988) discloses an oxymethylene homopolymer resin shaped article in which both a formation of spherulites having a very small diameter and an increase in the crystallinity have been achieved by utilizing the self-nucleating phenomenon. However, the flexural modulus of the shaped article disclosed in this prior art document is only 3.54 GPa. That is, this shaped article is not satisfactory with respect to the improvement in flexural modulus. From the molding conditions described in this prior art document, the present inventors have presumed that the reason why the oxymethylene homopolymer resin shaped article disclosed in this prior art document is not satisfactory with respect to the improvement in flexural modulus is because the growth of the crystal structure is unsatisfactory, especially the crystal structure which is evaluated in terms of an average crystallite size. As a result, a satisfactorily large average crystallite size cannot be obtained.